The long-term objectives of this project are to provide a more complete understanding of the factors contributing to the maintenance of an adequate venous return, and therefore cardiac filling pressure, in humans in conditions of circulatory and thermal strain. Specific aims are to relate changes in cutaneous vasomotor and venomotor tone to changes in arterial and central venous pressure in human subjects during conditions of high circulatory and thermal demand. This will be accomplished by having subjects exercise in the heat and by exposing resting subjects to lower body negative pressure, neck suction and facial immersion in ice water at different ambient temperatures and in condition in which blood volume has been manipulated prior to the experiment. Another specific aim of the project is to examine the Starling forces that determine capillary filtration dynamics in unfit, fit and heat acclimatized human subjects in order to identify the physiological changes that contribute to a relatively high cardiac filling pressure in the latter conditions and thereby provide resistance to syncope and other heat-related disorders. Another aim of present project is to study the effects of thiazide diuretics on blood volume over time, with the ultimate objective of determining whether the physiological adaptations to diuretic use mimic the physiological adpatations to diuretic use mimic the physiological adaptations to hear and exercise exposure. Another objective of this project is to evaluate the thermal and circulatory regulations in mild hypertensive individuals who are under control due to use of thiazide or Beta-adrenergic blocking drugs. This has great practical value in determining what limiations these people have for exercise and heat exposure. A final goal is the study of these same regulations in people who have had heart transplants and therefore no innervation to the heart and no low pressure baroreflex capability. Besides having practical significance, such study allows the separation of low from high pressure baroreflexes and thus and ability to gain further understanding of the control of the circulation.